This invention relates to the art of antenna systems for broadcast radios and communications equipment located in vehicles, and more particularly to a new and improved disguised antenna system with tuning or matching network.
An important area of use of the present invention is disguised antenna systems for vehicles containing both standard broadcast radios and communications equipment such as transceivers for automatic vehicle location, surveillance, law enforcement and similar functions. By disguised it is meant that the antenna and its mounting to the vehicle maintain the outward visible appearance of a standard radio broadcast antenna so as not to reveal the presence of communications equipment and the like in the vehicle.
A basic disguised antenna system includes a standard broadcast antenna mounted to a vehicle by means of a base, a tuning or matching network for matching the impedance of the broadcast radio and communication equipment such as a transceiver in the vehicle to the antenna on the outside of the vehicle and a broadcast coupler for providing isolation between the broadcast radio and the communications equipment.
The present invention provides a new and improved tuning or matching network which resonates the original equipment manufacturer""s antenna as supplied on the vehicle. The antenna system must not only continue to function as did the unmodified antenna providing normal AM and FM reception, but must also present a low SWR (Standing Wave Ratio) to a transmitting and/or receiving device. In an application involving two separated frequency spectrums, that are separated by several megahertz, a broadband approach that would function in all cases is not likely. The broadband approach would also be more costly. Since the automotive industry produces as many units as it does, cost is a major concern.
In an embodiment of the present invention a dual resonance will be required to achieve the desired results to provide the low SWR to the receiver and transmitter. For example, the receive passband can be located about ten megahertz below the transmit passband or vice versa. Actually, the present invention has been successful for passbands as close as 1-2% or as a remote as about 300% (such as 150 MHz and 450 MHz). All of this must be done while not changing the outward appearance of the original equipment manufacturer""s antenna. Since the original equipment manufacturer""s antenna is not a resonant length at either passband some form of matching or tuning network must be employed. The foregoing is accomplished according to the present invention by a matching or tuning network including a multiple stub tuner. The matching/tuning network for use with an original manufacturer""s antenna and including a multiple stub tuner according to the present invention both tunes the antenna for communications frequencies when combined with a broadcast coupler (which will become part of the tuner section) and separates the broadcast signals from the communications signals.
The following detailed description of the invention, when read in conjunction with the accompanying drawings, is in such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains, or with which it is most nearly connected, to make and use the invention.